Continuous wave

"A continuous wave or continuous waveform (CW) is an electromagnetic wave of constant amplitude and frequency. ... Continuous-wave radio was called radiotelegraphy because like the telegraph, it worked by means of a simple switch to transmit Morse code. However, instead of controlling the electricity in a cross-country wire, the switch controlled the power sent to a radio transmitter. This mode is still in common use by amateur radio operators."[1]

"In common usage and practice, C-W designates radiotelegraphy transmission using an unmodulated R-F carrier wave, turned on and off with a telegraph key to form the dits and dahs of Morse code. The 13-series transmitters were originally designed for voice operation only. The one aboard NR16020 was modified for C-W operation, primarily because of the requirement to operate on 500 KHz. This involved two major changes: alteration to the control circuitry; and the addition of a keying relay."

In order to hear when the CW transmitter has been keyed, the receiver must have its own tone generator that beeps when the set receives a transmission.